Surfactant-polymer compositions for enhancing the stability of viscoelastic-surfactant based fluid

ABSTRACT

Compositions for increasing the thermal and pressure stability of well fluids viscosified using viscoelastic surfactants, the compositions including an effective amount of an oligomeric or polymeric compound that has a thermally stable backbone structure and at least one pendent viscoelastic surfactant functional group. Preferred compositions for increasing the stability of well fluids viscosified using monomeric viscoelastic surfactants include an effective amount of an oligomeric or polymeric compound that has a thermally stable backbone structure and a multiplicity of pendent viscoelastic surfactant functional groups attached to said backbone structure through relatively long hydrocarbon chains, 1 to 18 carbons in length.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/319,575, filed Sep. 25, 2002, the entire contents ofwhich are incorporated by reference herein.

BACKGROUND

[0002] When drilling or completing wells in earth formations, variousfluids typically are used in the well for a variety of reasons. For thepurposes of the present disclosure, such a fluid will be referred to asa “well fluid.” Common uses for well fluids include: lubrication andcooling of drill bit cutting surfaces while drilling generally ordrilling-in (i.e., drilling into a targeted formation), transportationof cuttings to the surface, controlling formation pressure to preventblowouts, maintaining well stability, suspending solids in the well,minimizing fluid loss into and stabilizing the formation through which awell is being drilled, fracturing the formation in the vicinity of awell, displacing the fluid within a well with another fluid, cleaning awell, testing a well, emplacing spacer or fluid loss control pills atvarious points in the displacement, completion, or work-over process,emplacing a packer fluid in the completed wellbore during production,preparing the well for abandonment, abandoning the well or, otherwisetreating the well or the formation. A commonly used type of well fluidis based on water-based solutions including brines. Brines, such asCaBr₂ brine, are commonly used as well fluids because of the ability tocontrol the density of the solution over a wide density range. Furtherthe brines are typically substantially free of suspended solids andbrines typically do not damage the more common types of subterraneanformations.

[0003] When drilling progresses to the level of penetrating ahydrocarbon-bearing formation, special care may be required to maintainthe stability of the wellbore. Examples of formations in which problemsoften arise are highly permeable and/or poorly consolidated formationand thus a technique known as “under-reaming” may be employed. Inconducting the under-reaming process, the wellbore is drilled topenetrate the hydrocarbon-bearing zone using conventional techniques. Acasing generally is set in the wellbore to a point just above thehydrocarbon-bearing zone. The hydrocarbon-bearing zone then may bere-drilled to a wider diameter, for example, using an expandableunder-reamer that increases the diameter of the wellbore. Under-reamingusually is performed using special “clean” drilling fluids. Typicallythe “clean” drilling fluids used in under-reaming are aqueous, densebrines that are viscosified with a gelling and/or cross-linked polymerto aid in the removal of formation cuttings. The expense of such fluidslimits their general use in the drilling process.

[0004] When the target subterranean formation has a high permeability asignificant quantity of the drilling fluid may be lost into theformation. Once the drilling fluid is lost into the formation, itbecomes difficult to remove. Removal of the aqueous based well fluids isdesired to maximize the production of the hydrocarbon in the formation.It is well known in the art that calcium- and zinc-bromide brines canform highly stable, acid insoluble compounds when reacted with theformation rock itself or with substances contained within the formation.These reactions often may substantially reduce the permeability of theformation to any subsequent out-flow of the desired hydrocarbons. Asshould be well known in the art, it is widely and generally acceptedthat the most effective way to prevent such damage to the formation isto limit fluid loss into the formation. Thus, providing effective fluidloss control is highly desirable to prevent damaging thehydrocarbon-bearing formation. For example such damage may occur during,completion, drilling, drill-in, displacement, hydraulic fracturing,work-over, packer fluid emplacement or maintenance, well treating, ortesting operations.

[0005] Techniques that have been developed to control fluid loss includethe use of fluid loss control “pills.” As the term is used in thisdisclosure a “pill” is a quantity of fluid added to the well fluid so asto temporarily change the properties of the well bore fluid at or near aspecific point in the well bore. Significant research has been directedto determining suitable materials for the fluid loss pills, as well ascontrolling and improving the properties of the fluid loss pills.Typically, fluid loss pills work by enhancing filter-cake buildup on theface of the formation to inhibit fluid flow into the formation from thewellbore; however the fluids in accordance with the claimed subjectmatter are effective by developing extremely high viscosity in theenvironment at and just within the face of the formation to inhibitfluid flow into the formation from the wellbore. Because of the hightemperatures, high shear (caused by the pumping and placement of thepill), high pressures, and low pH to which well fluids may be exposed(i.e., “stress conditions”), synthetic polymeric materials typicallyused to form fluid loss pills and to viscosify the well fluids tend todegrade rather quickly.

[0006] One class of viscosifiers commonly used in the petroleum industrycomprises polymeric structures starting with molecular weights ofhundreds of thousands to several million grams per mole. These large,chemically bonded structures are often crosslinked to further increasemolecular weight and effective viscosity per gram of polymer added tothe fluid. These large molecules are quite stable under the thermalconditions typically encountered in a subterranean reservoir. However,this thermal stability is believed to contribute to decreased wellproductivity. As a result, expensive and often corrosive breakers havebeen designed to destroy the molecular backbone of these polymericstructures. These breakers are typically oxidizers or enzymes and are atbest only partially effective with typical reservoir cleanup less than80% complete and more usually much less than 50% complete. It is alsoreported in the literature that the long term stability of polymericbased thickening agents is shortened by the high temperature, highshear, high pressures, and low pH to which well fluids may be exposed(i.e., “stress conditions”).

[0007] Viscoelastic surfactants are commonly used in the petroleumindustry as an alternative to the above mentioned polymeric thickeningagents. Viscoelastic surfactants are relatively small molecules witheach molecule being typically less than 500 grams per mole (i.e.,molecular weight less than 500). These small molecules will associateunder certain conditions to form structures which resemble the polymermolecules but which are not stable structures. The individual moleculesof surfactant begin to associate to form rod-like orspiraling-cylinder-like micelles. These micelle structures are always inan equilibrium state of breaking and reforming. As dynamic structures,these polymer-shaped micelles are readily destroyed by shear, presenceof hydrocarbons or increased temperature. While these features aredesirable especially in a hydrocarbon-bearing formation, there isminimal control over the conditions under which micelle breakup occurs.Therefore, under conditions of exposure to oil, high temperature, highshear, or other “stress conditions”, the viscoelastic surfactantsrapidly return to their original small independent spherical micellarstate. When the viscoelastic micelles are broken down to this smallindependent spherical micellar state, the desired viscous nature of thewell fluid is lost. In some cases the loss is temporary, in others theloss may be more permanent.

[0008] Presently there exists an unmet need for a simple, inexpensiveway to increase the thermal range for viscoelastic-surfactant-basedviscosifying agents used in downhole applications. Preferably, thisthermal extender would be applicable to variousviscoelastic-surfactant-based viscosifying agents.

SUMMARY

[0009] Upon review of the following disclosed and claimed subjectmatter, one of skill in the art should appreciate and understand thatone illustrative embodiment is a wellbore fluid that includes: anaqueous based continuous phase; a viscoelastic surfactant; and asurfactant-polymer compound soluble in an aqueous solution. Thesurfactant-polymer compound has a molecular structure including ahydrophobic backbone and a plurality of hydrophilic functional groupsattached to the hydrophobic backbone. The hydrophobic backbone is thereaction product of one or more molecules having polymerizable alkene oralkyne functional groups, for example an oligo- or poly-ethylenestructure. In contrast the hydrophilic functional groups can bezwitterionic surfactant functional groups, anionic surfactant functionalgroups, cationic surfactant functional groups, and nonionic surfactantfunctional groups. The illustrative embodiment is molecularly designedsuch that the combination of the viscoelastic surfactant andsurfactant-polymer compound forms micellar assemblies in the wellborefluid. In one illustrative and exemplary embodiment the acid form of thesurfactant-polymer compound has the structure:

[0010] in which x has a value of 2 to 300,000 and preferably x has avalue of 2 to 36 Alternatively, the surfactant-polymer compound can be asalt of oligo- or poly-(α-alkenyl -ω- orα-alkynyl-ω-quaternary-ammonio-N-N-dialkyl-N-alkylcarboxylate) or amixture further comprising a salt ofN-alkyl-N-carboxymethyl-N,N-dimethylammonium chloride. Anotheralternative and illustrative embodiment is where the surfactant-polymercompound is a salt of oligo- orpoly-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate) ora mixture further comprising a salt ofN-hexadecyl-N-carboxymethyl-N,N-dimethylammonium chloride. A thirdalternative is where the surfactant-polymer compound is a salt of oligo-or poly-(α-alkenyl-ω- or α-alkynyl-ω-quaternary-ammonio-N,N-dialkyl-N-alkylcarboxylate), or is a salt ofoligo- orpoly-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate).

[0011] Additional embodiments of the claimed subject matter includezwitterionic surfactant heads such that the polymers or oligomers havethe following structures:

[0012] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0013] and in which R₁₀, R₁₁, R₁₂=H or CH₃, and t=1 to 16, u=6 to 12,v=1 to 18, w=1 to 3, and x+y+z=3 to 300,000 and S₁=CO₂ ⁻ or SO₃ ⁻. In apreferred illustrative embodiment, t=12 to 16, μ=6 to 12; v=12 to 18,w=1 to 3, x=0 to 10,000, y=2 to 300,000 and z=0 to 10,000

[0014] Alternatively the oligomer or polymer compound can be cationic inthe surfactant head and thus have a structure such as:

[0015] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0016]  and in which R₁₀, R₁₁, R₁₂ =H or CH₃, t=1 to 16, μ=6 to 12, v=1to 18, and x+y+z=3 to 300,000. An especially preferred and illustrativeembodiment includes an oligomer or polymer in which t=12 to 16, u=6 to12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000, and z=0 to10,000.

[0017] In yet another illustrative embodiment, the oligomer or polymercan have a molecular structure that includes an anionic surfactantfunctional group such as:

[0018] where R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0019]  in which R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to18, x+y+z=3 to 300,000, and S₁=CO₂ ⁻or SO₃ ⁻. In one such illustrativeembodiment, it is preferred that t=12 to 16, u=6 to 12, v=12 to 18, x=0to 10,000, y=2 to 300,000, and z=0 to 10,000.

[0020] Further as noted above, the illustrative oligomer or polymer canhave a nonionic surfactant group and preferably has a molecularstructure such as:

[0021] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0022] and in which, R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1to 18, w=1 to 12, and x+y+z=3 to 300,000. In such instances, a preferredillustrative embodiment is achieved when t=12 to 16, u=6 to 12, v=12 to18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000, and z=0 to 10,000.

[0023] Fundamentally the polymeric backbone can be saturated as noted inthe above illustrative examples or unsaturated. In such illustrativeembodiments, the oligomer or polymer has a back bone structure such asthe following:

[0024] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃. As for the R₇, R₈ andR₉ groups, these may be the same as those disclosed above. Thus one ofskill in the art should appreciate that zwitterionic, cationic, anionicand nonionic surfactant groups may be attached to the unsaturatedbackbone structure shown above and that such compounds are illustrativeof the disclosed and claimed subject matter.

[0025] As discussed above, the novel oligomers and polymers taught inaccordance with the claimed subject matter contain chemical functionalgroups that are similar to those found in conventional viscoelasticsurfactants and thus are highly compatible with conventionalviscoelastic surfactant systems. Accordingly, the claimed subject matterteaches blends of the viscoelastic oligomers or polymers withconventional viscoelastic surfactant systems. The conventionalviscoelastic-surfactant-based fluids useful for the claimed subjectmatter are those in the following references, all of which areincorporated herein by reference—Canadian Patent 1,298,697, U.S. Pat.Nos. 4,615,825, 4,695,389, 4,725,372, 5,258,137, 5,551,516, 5,691,292,5,964,295, 5,965,502, 5,979,555, 5,979,557, 6,140,277, 6,194,355,6,194,356, 6,211,120, 6,232,274, 6,239,183, Paper SPE 17,168, Paper SPE30,098, Paper SPE 30,114, Paper SPE 30,458, Paper SPE 31,114, Paper SPE38,622, Paper SPE 56,467, Paper SPE 57,432, Paper SPE 59,478, and PaperSPE 60,322. Of these, the preferred viscoelastic-surfactant-based fluidsare those based on anionic, cationic, or zwitterionic surfactants ormixtures of anionic and nonionic surfactants or mixtures of cationic andnonionic surfactants or mixtures of zwitterionic and nonionicsurfactants. And of these, the particularly preferredviscoelastic-surfactant-based fluids are those based on zwitterionicsurfactants or mixtures of zwitterionic and nonionic surfactants. Inboth the preferred viscoelastic-surfactant-based fluids, and in theparticularly preferred viscoelastic-surfactant-based fluids, a minorityamount of an additional surfactant, termed a “co-surfactant”, such as,for example, 2-ethylhexanol or SDBS may optionally be employed. Theviscoelastic oligomers or polymers of the claimed subject matter may becreated in the presence of conventional viscoelastic surfactant systemsor may be synthesized in separate steps, optionally processed or dried,and then subsequently admixed into a solution of conventionalviscoelastic surfactants.

[0026] It has been found that the oligomers or polymers of theillustrative drilling fluids can be crosslinked with polyvalent metalions, formaldehyde, or glutaraldehyde. In one such embodiment, thepolyvalent metal ions are selected from: Fe²⁺, Cd²⁺, Co²⁺, Ca²⁺, Cu²⁺,UO₂ ²⁺, PbO²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ce³⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺and mixturesthereof.

[0027] One of skill in the art should also appreciate that the disclosedand claimed subject matter includes a method of making a wellbore fluidas is disclosed herein. In one such illustrative embodiment, the methodinvolves the blending of an aqueous fluid phase, a viscoelasticsurfactant, a water-soluble inorganic salt, and an oligomer or polymersoluble in the aqueous salt solution. The oligomer or polymer includes ahydrophobic oligomeric or polymeric backbone made from theoligomerization or polymerization of alkene or alkyne monomer groups, ormixtures thereof. The oligomer or polymer further comprises surfactantfunctional groups attached to the hydrophobic backbone, wherein theoligomer or polymer is hydrophilic in the surfactant functional groupsand hydrophobic in the backbone hydrocarbon chain. Thus the oligomer orpolymer has a molecular structure that promotes the formation ofmicellar assemblies such that the oligomers or polymers developviscoelastic character prior to a polymerization step.

[0028] The claimed subject matter also encompasses a method of drillinga subterranean well as well as other sues for the wellbore fluid thatshould be apparent to one of skill in the art.

[0029] Further details regarding the claimed subject matter can be foundin the following description.

DESCRIPTION

[0030] The claimed subject matter relates to compositions for increasingthe thermal durability of viscoelastic-surfactant-based well fluids.More specifically, the claimed subject matter relates to the fields offluid rheology, thickeners, viscosifiers, viscoelastic fluids,viscoelastic surfactant fluids, drilling fluids, well fracturing fluids,well treatment fluids and fluid control pills. Further, the claimedsubject matter relates to increasing the thermal and pressure stabilityof well fluids viscosified using viscoelastic surfactants by includingan effective amount of a surfactant-oligomeric or surfactant-polymericcompound that has a thermally stable backbone structure and viscoelasticsurfactant appendages. As the term is used in the present disclosure,“effective” simply means an amount sufficient to raise the temperaturestability of the viscoelastic-surfactant based well fluid system by ameasurable amount.

[0031] In accordance with one illustrative embodiment of the claimedsubject matter, a sufficient quantity of at least onesurfactant-oligomeric or surfactant-polymeric compound that is solublein an aqueous salt solution is employed to affect the desired viscosity.In the claimed subject matter, the molecules of thesurfactant-oligomeric or surfactant-polymeric compound have ahydrophobic oligomeric or polymeric backbone made preferably from theoligomerization or polymerization of alkene and/or alkyne groups. As theterm is used herein, “thickener” and surfactant-oligomeric orsurfactant-polymeric compound are used interchangeably and are intendedto mean the compounds substantially described and claimed herein. Thethickener of the claimed subject matter also includes chemicalfunctional groups that are structurally similar to prior artviscoelastic surfactants and therefore these molecules exhibit similarchemical characteristics of prior art viscoelastic surfactants. Thus thehydrophobic backbone is chemically linked to and thus rendered at leastin part hydrophilic by the presence of these chemical functional groups.One such illustrative compound is the product of the oligomerizationreaction of a monomer such as the sodium salt ofN-N-dimethyl-N-methylcarboxylate-N-1-hepten-7-ammonium chloride to givethe sodium salt ofoligo-(1-hepten-7-quaternary-ammonio-N-N-dimethyl-N-methylcarboxylate).The resulting oligomer is believed to have the simplified structure asindicated below in the acid form rather than the sodium-salt form:

[0032] in which x will have a value from about 2 to several hundredthousand, preferably from about 2 to several dozen. The monomer may beprepared, for example, by the reaction of N-hexadecyl-N,N-dimethylaminewith chloroacetic acid to produce N-hexadecyl-N-methylcarboxylicacid-N,N-dimethylammonium chloride. Upon neutralization with sodiumhydroxide, the final product is the zwitterionic betaine which is thesodium salt of N-hexadecyl-N-carboxymethyl-N,N-dimethylammoniumchloride—it has a negative charge on the carboxyl group and the sodiumcation associated with it as a counter ion, and a positive charge on thequaternary amine group and the chloride anion associated with it as acounter ion. Alternatively, the sodium and chloride counter ions may beseparated therefrom, leaving the negatively charged carboxyl group andthe positively charged quaternary amine group as counter ions for eachother.

[0033] The sodium salt ofoligo-(1-hepten-7-quaternary-ammonio-N-N-dimethyl-N-methylcarboxylate)is an example of a salt of oligo- orpoly-(α-alkenyl-ω-quaternary-ammonio-N-N-dialkyl-N-alkylcarboxylate).This example begins to define the terms α-alkenyl-ω- and α-alkynyl-ω-,wherein the “α-” designation denotes a location at or near one end ofthe hydrocarbyl chain, such as, for example at the 1-, 2-, or 3-positionand wherein the “ω-” designation denotes a location at or near theopposite end of the hydrocarbyl chain from the α-position, such as, forexample at the very end of the hydrocarbyl chain, at one carbon groupaway from the very end of the hydrocarbyl chain, or at two carbon groupsaway from the very end of the hydrocarbyl chain. An α-alkene and anα-alkyne are defined similarly. An α-alkenyl-ω-carboxylate salt and anα-alkynyl-ω-carboxylate salt are defined in a parallel manner. Anα-alkenyl-ω-N,N,N-trialkylammonium salt and anα-alkynyl-ω-N,N,N-trialkylammonium salt are defined similarly. An α,α+2-alkadienyl-ω-carboxylate salt and an α, α+2-alkadiynyl-ω-carboxylatesalt are defined in a parallel manner.

[0034] In one illustrative embodiment, a monomer like the sodium salt ofN-carboxymethyl-N,N-dimethyl-N-1-hepten-7-ammonium chloride could bemixed into the solution of the conventional rod-shaped or spaghetti-likeor spiraling-cylinder-like micelles of the viscoelastic surfactant thatis the sodium salt of N-hexadecyl-N-carboxymethyl-N,N-dimethylammoniumchloride. This illustrative monomer, the sodium salt ofN-carboxymethyl-N,N-dimethyl-N-1-hepten-7-ammonium chloride, differsonly in minor ways from the sodium salt ofN-hexadecyl-N-carboxymethyl-N,N-dimethylammonium chloride. Accordingly,the monomer will be readily subsumed into the conventional rod-shaped orspaghetti-like or spiraling-cylinder-like micelles, whereupon one cantake well-known steps to initiate the oligomerization or polymerizationof the monomer to produce the sodium salt of oligo- orpoly-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate).This oligomer or polymer will be inherently hydrophilic in itszwitterionic functional groups and hydrophobic in the hydrocarbon chainsthat link all the zwitterionic functional groups to each other. Theoligomer or polymer is believed to be structurally quite similar to theviscoelastic surfactant molecules in the well fluid and therefore issoluble or dispersible in the well fluid solution. Similar oligomers orpolymers are likewise soluble or dispersible in other viscoelasticsurfactant solutions such as 10% XE862 (a product that is commerciallyavailable from Schlumberger) solution with 0.3% sodium dodecylbenzenesulfonate (SDBS) in 13.5 pound per barrel CaBr₂-based brine. Upon mixingthe oligomer or polymer with the viscoelastic-surfactant-based fluid,the oligomer or polymer is believed to be subsumed into the rod-shapedor spaghetti-like micelles, whereupon, sitting inside these rod-shapedor spaghetti-like or spiraling-cylinder-like micelles is one or moresuch oligomer or polymer molecules and when configured in this fashion,the oligomer or polymer molecules impart greater thermal stability tothe micelles, greater resistance to shear stress and other stressconditions acting upon a fluid loss pill—including, for example,exposure to oil, high shear in pumping and placement, high temperature,high differential pressure, and low pH.

[0035] Another illustrative embodiment of the compounds of the claimedsubject matter includes the oligomeric or polymeric products of theco-oligomerization or co-polymerization of two different monomers suchas 1-heptene and the sodium salt ofN-carboxymethyl-N,N-dimethyl-N-1-hepten-7-ammonium chloride.Alternatively, 1-heptene can be first co-oligomerized or co-polymerizedwith N,N-dimethyl-l-hepten-7-amine and in a subsequent reaction theamine groups are reacted with 1-chloro-propane-3-sulfonic acid and thencaustic to produce oligo- orpoly-(1-heptene-co-1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate).The resulting oligomer or polymer is believed to have the simplifiedstructure as indicated below:

[0036] in which x and y will have values dependent upon the molar ratioof reactants added to the polymerization reaction and the sum of x and ywill have a value of about 2 to several hundred thousand, preferablyfrom about 2 to several dozen. While this oligomer or polymer will beinherently hydrophilic in its zwitterionic functional groups, there aretwo sources of hydrophobicity: in the 1-heptene co-oligomer orco-polymer species and in the hydrocarbon chains that link all thezwitterionic functional groups to each other. By varying the molar ratioof the monomers present during the oligomerization or polymerizationreaction, the hydrophobicity and viscoelastic surfactant properties ofthe resulting oligomer or polymer may be controlled. The oligomer orpolymer contains a point of structural similarity to the small moleculeviscoelastic surfactants and therefore is soluble or dispersible in suchconventional viscoelastic surfactant solutions. Similar oligomers orpolymers are likewise soluble or dispersible in the 10% XE862 solutionwith 0.3% SDBS in 13.5 pounds per gallon CaBr₂-based brine discussedabove. Upon admixing the polymer with the viscoelastic-surfactant-basedfluid, the polymer is believed to be subsumed into the rod-shaped orspaghetti-like or spiraling-cylinder-like micelles, whereupon, sittinginside these rod-shaped or spaghetti-like or spiraling-cylinder-likemicelles is one or more such polymer molecules and when configured inthis fashion, the polymer molecules impart greater thermal stability tothe micelles, greater resistance to shear stress and other stressconditions acting upon a fluid loss pill—including, for example,exposure to oil, high shear in pumping and placement, high differentialpressure, and low pH.

[0037] Yet a third illustrative embodiment of the compounds of theclaimed subject matter includes the co-oligomerization orco-polymerization of three or more co-monomers. For example, one couldco-oligomerize or co-polymerize 1-heptene, 1-heptene-6,7-diol andN,N-dimethyl-1-hepten-7-amine. In such an illustrative embodiment, theamine groups are reacted with 1-chloro-propane-3-sulfonic acid and thencaustic to produce oligo- or poly- (1-heptene -co-1-heptene-6,7-diol-co-1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate). Theresulting oligomer or polymer is believed to have the simplifiedstructure as indicated below:

[0038] in which x, y and z will have values dependent upon the molarratio of reactants added to the polymerization reaction and the sum ofx, y, and z will have a value of about 3 to several hundred thousand,preferably from about 3 to several dozen. As with the other oligomers orpolymers of the claimed subject matter, the above illustrative oligomeror polymer is soluble or dispersible in conventional viscoelasticsurfactant well fluids such as the 10% XE862 solution with 0.3% SDBS in13.5 pounds per gallon CaBr₂-based brine discussed above. Upon admixingthe illustrative oligomer or polymer with such aviscoelastic-surfactant-based fluid, the oligomer or polymer is believedto be subsumed into the rod-shaped spaghetti-like orspiraling-cylinder-like micelles, whereupon, sitting inside theserod-shaped spaghetti-like or spiraling-cylinder-like micelles is one ormore such oligomer or polymer molecules and when configured in thisfashion, the oligomer or polymer molecules impart greater thermalstability to the micelles, greater resistance to shear stress and otherstress conditions acting upon a fluid loss pill—including, for example,exposure to oil, high shear in pumping and placement, high differentialpressure, and low pH. Additionally, the co-monomer 1-heptene-6,7-diolincorporates a vicinal diol functionality into the oligomer or polymer.Surface forces should cause the vicinal diol functionality to presentitself at the outer surface of any rod-shaped micelle in which it hasbecome subsumed.

[0039] The vicinal diol functionality provides the chemical functionalgroup that renders the oligomers or polymers readily crosslinkable withpolyvalent metal ions or complexes such as, for example, a borate,titanate, or zirconate crosslinker as taught in U.S. Pat. No. 5,062,969,(2) divalent, trivalent, or tetravalent cations such as, for example,Fe²⁺, Cd²⁺, Co²⁺, Ca²⁺, Cu²⁺, UO₂ ²⁺, PbO²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ce³⁺,Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, and the like, (3) complexes of or other moietiescontaining the crosslinkers listed above in the first two categories,such as, for example, the tetrammine complex of the Cu²⁺cation, thecarbonate anion complexes of the UO₂ ²⁺ cation, UO_(2 l (CO) ₃)₂ ²⁻ andUO_(2 l (CO) ₃)₃ ⁴⁻, or the triethanolamine complex of the Ti⁴⁺ cation,(4) so-called “organic crosslinkers” such as, for example, formaldehyde,and glutaraldehyde, and (5) mixtures of the crosslinkers listed above inthe first four categories and/or reaction products therefrom. Thus it iscontemplated that one of skill in the art could, if desired, crosslinkthe illustrative oligomers or polymers with polyvalent metal ions,complexes, organic crosslinkers, or mixtures thereof, as describedabove. This is believed to lead effectively to the crosslinking of theviscoelastic surfactant assemblies in which the oligomers or polymershave been subsumed.

[0040] One of skill in the art should at this point appreciate that theco-oligomerization or co-polymerization reactions disclosed above resultin the random placement of each co-monomer along the vinyl backbone ofthe illustrative oligomers or polymers. However, it is contemplated thatblock co-oligomerization or co-polymerization could also be used toachieve substantially similar results. Alternatively, blockco-oligomerization or co-polymerization could also be used to carefullytailor the properties of the resulting oligomers or polymers made inaccordance with the claimed subject matter. One of skill in the artshould understand and appreciate that by systematically controlling themolar ratio of and concentration of monomers present in during theoligomerization or polymerization process, the order of addition, thetemperature and duration of oligomerization or polymerzation and theinitiators and catalysts used and their concentrations, the propertiesof the compounds of the claimed subject matter can be carefullycontrolled and tailored.

[0041] Other oligomers or polymers formulated in accordance with andillustrative of the claimed subject matter include such oligomers orpolymers as oligo- or poly-(N-carboxymethyl-N,N-diallyl-N-methylammonium chloride), oligo- or poly-(N,N-diallyl-N,N-dimethyl ammoniumchloride-co-N-carboxymethyl-N,N-diallyl-N-methyl ammonium chloride),oligo- or poly-(1-butene-co-N-carboxymethyl-N,N-diallyl-N-methylammonium chloride), oligo- or poly- (1-butene-co-1-pentene-4,5-diol-co-N-carboxymethyl-N,N-diallyl-N-methyl ammonium chloride), oligo- orpoly-alkenyl or -alkynyl DMAPA amides (see Figure below), oligo- orpoly-alkenyl or -alkynyl DMAPA quats, and oligo- or poly-alkenyl or-alkynyl tallow amine quaternary amines such as

C₁₂₋₂₂ DMAPA Amide

[0042]

C₁₂₋₂₂ DMAPA Amide Quaternary Amine

[0043]

Tallow Amine Quatemary Amine

[0044]

[0045] and the like.

[0046] Except for the oligo- or poly-alkenyl or -alkynyl DMAPA amides,oligo- or poly-alkenyl or -alkynyl DMAPA quaternary amines, and oligo-or poly-alkenyl or -alkynyl tallow amine quaternary amines, oligomers orpolymers of this type may be unlike those of the previous illustrativeembodiments of the claimed subject matter in that they are probably notcapable of effectively vacuuming up the micelles and packing them aboutthemselves until there are enough surfactant molecules present in thevicinity of the oligomer or polymer molecule so that the sphericalmicelles can merge into a single rod-shaped or spaghetti-like orspiraling-cylinder-like micelles with the oligomer or polymer moleculesubsumed within the rod-shaped or spaghetti-like orspiraling-cylinder-like micelles. Polymers or oligomers in accordancewith the present illustrative embodiment of the claimed subject matterare designed to be compatible with CaBr₂-, CaBr₂/CaCl₂-, ZnBr₂/CaBr₂-,and ZnBr₂/CaBr₂/CaCl₂-based brines.

[0047] Other oligomers or polymers formulated in accordance with andillustrative of the claimed subject matter include oligomers or polymerssuch as oligo- or poly-(ethylene-co-N,N-diallyl-N,N-dimethyl ammoniumchloride), oligo- or poly-(1-butene-co-N,N-diallyl-N,N-dimethyl ammoniumchloride), oligo- orpoly-(1-butene-co-1-pentene-4,5-diol-co-N,N-diallyl-N,N-dimethylammonium chloride), and the like. Polymers or oligomers of this type areprobably not sufficiently compatible with CaBr₂-, CaBr₂/CaCl₂-,ZnBr₂/CaBr₂-, and ZnBr₂/CaBr₂/CaCl₂-based brines; however, they expandthe scope of the claimed subject matter into useful dimensions to theextent that these polymers should be quite compatible with NaCl-, NaBr-,NaBr/NaCl-, CsBr/KBr-, and CsBr/KBr/NaCl-based brines and the like.Poly- oroligo-(1-butene-co-1-pentene-4,5-diol-co-N,N-diallyl-N,N-dimethylammonium chloride) incorporates a vicinal diol functionality, and so oneof skill in the art should appreciate that this polymer or oligomer maybe crosslinkable with polyvalent metal ions as disclosed above. As notedabove, when the oligomers or polymers are crosslinked with polyvalentmetal ions, we also effectively crosslink the viscoelastic surfactantassemblies in which the oligomers or polymers have been subsumed.

[0048] Additional embodiments of the claimed subject matter includezwitterionic surfactant heads such that the polymers or oligomers havethe following structures:

[0049] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0050] and in which R₁₀, R₁₁, R₁₂ =H or CH₃, and t=1 to 16, u=6 to 12,v=1 to 18, w=1 to 3, and x+y+z=3 to 300,000 and S₁=CO₂ ⁻ or SO₃ ⁻. In apreferred illustrative embodiment, t=12 to 16, u=6 to 12; v=12 to 18,w=1 to 3, x=0 to 10,000, y=2 to 300,000 and z=0 to 10,000

[0051] Alternatively the oligomer or polymer compound can be cationic inthe surfactant head and thus have a structure such as:

[0052] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0053] and in which R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to18, and x+y+z=3 to 300,000. An especially preferred and illustrativeembodiment includes an oligomer or polymer in which t=12 to 16, u=6 to12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000, and z=0 to10,000.

[0054] In yet another illustrative embodiment, the oligomer or polymercan have a molecular structure that includes an anionic surfactantfunctional group such as:

[0055] where R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0056] in which R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18,x+y+z=3 to 300,000, and S₁=CO₂ ⁻ or SO₃ ³¹. In one such illustrativeembodiment, it is preferred that t=12 to 16, u=6 to 12, v=12 to 18, x=0to 10,000, y=2 to 300,000, and z=0 to 10,000.

[0057] Further as noted above, the illustrative oligomer or polymer canhave a nonionic surfactant group and preferably has a molecularstructure such as:

[0058] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

[0059] and in which, R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1to 18, w=1 to 12, and x+y+z=3 to 300,000. In such instances, a preferredillustrative embodiment is achieved when t=12 to 16, u=6 to 12, v=12 to18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000, and z=0 to 10,000.

[0060] Fundementally the polymeric backbone can be saturated as noted inthe above illustrative examples or unsaturated. In such illustrativeembodiments, the oligomer or polymer has a back bone structure such asthe following:

[0061] in which R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃. As for the R₇, R₈ andR₉ groups, these may be the same as those disclosed above. Thus one ofskill in the art should appreciate that zwitterionic, cationic, anionicand nonionic surfactant groups may be attached to the unsaturatedbackbone structure shown above and that such compounds are illustrativeof the disclosed and claimed subject matter.

[0062] As discussed above, the novel oligomers and polymers taught inaccordance with the claimed subject matter contain chemical functionalgroups that are similar to those found in conventional viscoelasticsurfactants and thus are highly compatible with conventionalviscoelastic surfactant systems. Accordingly, the claimed subject matterteaches blends of the viscoelastic oligomers or polymers withconventional viscoelastic surfactant systems. The conventionalviscoelastic-surfactant-based fluids useful for the claimed subjectmatter are those in the following references, all of which areincorporated herein by reference—Canadian Patent 1,298,697, U.S. Pat.No. 4,615,825, 4,695,389, 4,725,372, 5,258,137, 5,551,516, 5,691,292,5,964,295, 5,965,502, 5,979,555, 5,979,557, 6,140,277, 6,194,355,6,194,356, 6,211,120, 6,232,274, 6,239,183, Paper SPE 17,168, Paper SPE30,098, Paper SPE 30,114, Paper SPE 30,458, Paper SPE 31,114, Paper SPE38,622, Paper SPE 56,467, Paper SPE 57,432, Paper SPE 59,478, and PaperSPE 60,322. Of these, the preferred viscoelastic-surfactant-based fluidsare those based on anionic, cationic, or zwitterionic surfactants ormixtures of anionic and nonionic surfactants or mixtures of cationic andnonionic surfactants or mixtures of zwitterionic and nonionicsurfactants. And of these, the particularly preferredviscoelastic-surfactant-based fluids are those based on zwitterionicsurfactants or mixtures of zwitterionic and nonionic surfactants. Inboth the preferred viscoelastic-surfactant-based fluids, and in theparticularly preferred viscoelastic-surfactant-based fluids, a minorityamount of an additional surfactant, termed a “co-surfactant”, such as,for example, 2-ethylhexanol or SDBS may optionally be employed. Theviscoelastic oligomers or polymers of the claimed subject matter may becreated in the presence of conventional viscoelastic surfactant systemsor may be synthesized in separate steps, optionally processed or dried,and then subsequently admixed into a solution of conventionalviscoelastic surfactants.

[0063] The aqueous well fluids made in accordance with the claimedsubject matter may optionally include a sufficient quantity of at leastone water-soluble inorganic salt to effect formation stability.Typically, water-soluble potassium and ammonium salts, such as potassiumchloride and ammonium chloride are employed. However, other shaleinhibition compounds may be utilized including organic amine basedcompounds and other known shale inhibition agents. Additionally, calciumchloride, calcium bromide and zinc halide salts may also be used toincrease the specific gravity (i.e., the density) of the solution.Formation stability and in particular clay stability are achieved at aconcentration of a few percent by weight and as such the density of thefluid is not significantly altered by the presence of the inorganic saltunless fluid density becomes an important consideration, at which point,heavier inorganic salts may be employed.

[0064] Other compounds useful in the claimed subject matter includeother viscosifiers, corrosion inhibitors, lubricants, pH controladditives, surfactants, solvents, and/or weighting agents, among otheradditives. Some typical brine-based well fluid viscosifying additivesinclude “natural” or biopolymers or derivatives thereof, such as, forexample, xanthan gum and hydroxyethyl cellulose (HEC) or syntheticpolymers and oligomers such as poly(ethylene glycol) (PEG), poly(diallylamine), poly(acrylamide), poly(aminomethylpropylsulfonate [AMPS]),poly(acrylonitrile), poly(vinyl acetate), poly(vinyl alcohol),poly(vinyl amine), poly(vinyl sulfonate), poly(styryl sulfonate),poly(acrylate), poly(methyl acrylate), poly(methacrylate), poly(methylmethacrylate), poly(vinylpyrrolidone), poly(vinyl lactam) and co-, ter-,and quater-polymers of the following co-monomers: ethylene, butadiene,isoprene, styrene, divinylbenzene, divinyl amine, 1,4-pentadiene-3-one(divinyl ketone), 1,6-heptadiene-4-one (diallyl ketone), diallyl amine,ethylene glycol, acrylamide, AMPS, acrylonitrile, vinyl acetate, vinylalcohol, vinyl amine, vinyl sulfonate, styryl sulfonate, acrylate,methyl acrylate, methacrylate, methyl methacrylate, vinylpyrrolidone,and vinyl lactam. Yet other viscosifiers include the clay-basedviscosifiers, especially laponite and other small fibrous clays such asthe polygorskites (attapulgite and sepiolite).

[0065] One of ordinary skill in the art should appreciate that the useof “inert” filler materials can be added to impart strength to a fluid.Examples of such materials include shredded rubber tires, shreddedbattery casings, peanut hulls, cotton seed hulls, woody material, andother plant fibers that should be well known to one of skill in the art.

[0066] Additional brine-based well fluid surfactant additives useful inthe claimed subject matter include nonionic surfactants, such asethoxylated nonylphenols containing about 6 to 20 moles of ethyleneoxide, or alkyl polyethyleneoxyalcohols, ethoxylated linear alcohols,ethoxylated tridecyl alcohols, ethoxylated phenols such as nonylphenolsand dodecylphenols and fatty dialkanol amides. Hydroxyethyl fatty aminesare also classified as nonionic surfactants, although at low pH, theymay take on some cationic character. Emulsifying surfactants includingoil soluble surfactants, such as fatty diethanolamides, sorbitan fattyacid esters, and ethoxylated sorbitan fatty acid esters such as sorbitanmonooleate and sorbitan sesqioleate, because of their limited solubilityin typical brines, may however be incidentally included in theformulation of other products—polymer solutions, emulsions, or slurries,corrosion inhibitors, lubricants, solvents, or weighting agents—that areused as additives to brine-based well fluids.

[0067] Other surfactants useful in the claimed subject matter are thoselisted in “McCutcheon's Emulsifiers and Detergents 1999: North AmericanEdition” (ISBN: 0944254624), incorporated herein by reference. They areclassified as anionic, nonionic, amphoteric, zwitterionic, alcohols,alkanolamides, alkanolamines, alkylaryl sulfonates, alkylaryl sulfonicacids, amine acetates, amine oxides, amines, sulfonated amines,sulfonated amides, betaine derivatives, block polymers, carboxylatedalcohols, alkylphenol ethoxylates, carboxylic acids, fatty acids,ethoxylated alcohols, ethoxylated alkylphenols, ethoxylated amines,ethoxylated amides, ethoxylated fatty acids, ethoxylated fatty esters,fatty esters, fluorocarbon-based surfactants, glycerol esters, glycolesters, heterocyclic surfactants, imidazolines, imidazoline derivatives,isethionates, lanolin-based derivatives, lechithin, lechithinderivatives, methyl esters, monoglycerides, monoglyceride derivatives,olefin sulfonates, phosphate esters, phosphorous organic derivatives,polyethylene glycols, polymeric surfactants (polysaccharides,polyacrylic acids, polyacrylamides), propoxylated alcohols, propoxylatedalkylphenols, propoxylated amines, propoxylated amides, propoxylatedfatty acids, propoxylated fatty esters, protein-based surfactants,quaternary surfactants, sarcosamine derivatives, silicone-basedsurfactants, soaps, sodium isethionate, sorbitan derivatives, sucroseand glucose esters and derivatives, sulfates and sulfonates of oils andfatty acids, sulfates and sulfonates of ethoxylated alkylphenols,sulfates of alcohols, sulfates of ethoxylated alcohols, sulfates offatty esters, sulfonates of hydrocarbons and petroleum, sulfosuccinatesand derivatives, taurates, thio and mercapto derivatives, and tridecyland dodecyl benzene sulfonic acids.

[0068] As mentioned earlier, the claimed subject matter concentrates onthose micellar assemblies that are dominated by (1) one-dimensionalextensions of the spherical micelle into rod-shaped or spaghetti-like orspiraling-cylinder-like micelles, (2) two-dimensional extensions of thespherical micelle into planar micelles or, more likely, stacks of planarmicelles (as in liquid crystals), or (3) vesicular micelles. Vesicularmicelles may be spherical at their very centers, but each center spherewill be surrounded by a second, third, and possibly more layers ofsurfactants. Moving outward radially from the central sphere ofsurfactant molecules, each successively outer layer of surfactantmolecules will be oriented oppositely from next inner-most layer.Vesicular micelles may also be rod-shaped at their very centers, buteach central rod-shaped assembly will be surrounded by a second, third,and possibly more layers of surfactants. A stack of planar micelles maybe thought of as the vesicular form of a single planar micelle. Theseconcepts comprise yet another illustrative embodiment of the claimedsubject matter.

[0069] The above-mentioned embodiments may also comprise oligomers orpolymers created by first carrying out a micellar polymerization toproduce an oligomer or polymer, and then optionally processing or dryingthe product, and then subsequently admixing it into a solution ofconventional surfactants—(1) non-viscoelastic or (2) non-viscoelasticand viscoelastic in nature. The micelles of the conventional surfactantswould, in preferred instances, subsume the oligomers or polymers intothe conventional surfactant micelles to create mixed micelles havingsuperior viscoelastic properties in comparison to those of equivalentsolutions not comprising the conventional surfactants and/or incomparison to those of equivalent solutions not comprising the oligomersor polymers.

[0070] It should also be appreciated by one of skill in the art that theclaimed subject matter also relates to methods of using fluid losscontrol pills and similar fluids containing the compounds of the claimedsubject matter, that can sustain stress conditions for extended periodsof time without significant fluid loss or loss of desirable Theologicalproperties. The stress conditions may include, for example, exposure tooil, high shear in pumping and placement, exposure to oxidizing breakers(including oxygen dissolved in the fluid), exposure to brines havinghigh divalent cation content, high temperature, high differentialpressure, low pH, extended time, and a combination of two or more ofsuch stress conditions. These pills and fluids are advantageouslyapplied in or in connection with drilling, drill-in, displacement,completion, hydraulic fracturing, work-over, packer fluid emplacement ormaintenance, well treating, testing, or abandonment.

[0071] Exemplary viscoelastic surfactant compositions of improvedstability in fluid loss control in accordance with the claimed subjectmatter are given in the following examples. The following examples areincluded to demonstrate preferred embodiments of the claimed subjectmatter. It should be appreciated by those of skill in the art that thetechniques disclosed in the examples which follow represent techniquesdiscovered by the inventors to function well in the practice of theclaimed subject matter, and thus can be considered to constitutepreferred modes for its practice. However, those of skill in the artshould, in light of the present disclosure, appreciate that many changescan be made in the specific embodiments which are disclosed and stillobtain a like or similar result without departing from the scope of theclaimed subject matter.

EXAMPLE 1

[0072] The oligomerization of the surfactantN-dodecene-1-yl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride:

[CH₂═CH(CH₂)₁₈][HO(CH₂)₂]₂CH₃N⁺Cl⁻

[0073] in an aqueous solution was achieved by joining a few of thecarbon-carbon double bonds by a free radical oligomerization reactionwithin the micelles using the following processes. A viscoelasticsurfactant solution is produced using 30 g/l of the surfactantN-dodecene-1-yl-N,N-bis(2-hydroxyethyl)-N-methylammonium chloride with40 g/l ammonium chloride. A volume of 100 ml of the viscoelasticsurfactant solution was placed in a bottle purged with oxygen-free, drynitrogen gas to remove any dissolved oxygen. After sufficient purging0.10 mg of the free radical initiator 2,2′-azo(bis-amidinopropane)dihydrochloride was added to the viscoelastic surfactant solution andmixed thoroughly. The surfactant solution was heated at 60° C. for 30minutes under an atmosphere of oxygen-free, dry nitrogen. The resultingoligomer can be thought of as related to oligo-ethylene, a relativelyshort-chained polyethylene, to which relatively long pendant surfactantgroups—[(CH₂)₁₈][HO(CH₂)₂]₂CH₃N⁺Cl⁻— are linked. Oligomerization of thesurfactant monomers in micelles resulted in the viscosity of the gelbecoming largely insensitive to contact with hydrocarbon. The viscosityof the surfactant gel was not materially altered by the oligomerizationof the surfactant monomers. The oligomerized surfactant gel retained itsgel strength after prolonged contact with water. A volume of 50 ml ofthe oligomerized viscoelastic surfactant solution was blended with 50 mlof the original un-oligomerized viscoelastic surfactant solution. Theresulting solution was blended with 8 volume % of n-hexane with nomaterial reduction in the viscosity of the mixture being observed,whereas mixing only 6 volume % of n-hexane with the originalun-oligomerized viscoelastic surfactant solution led to loss ofessentially all viscosity in the mixture. Advantageously, therefore,compositions formed in accordance with the claimed subject matterprovide viscoelastic surfactant fluids that are capable of controllingfluid loss, and that are capable of dissolving in fluid without leavingsubstantial amounts of residue. Further, the compositions are capable ofcontrolling fluid loss at temperatures up to about 350° F. or higher,and may also control fluid loss for an extended period of at least 4days, and finally, do not interfere with other chemical additivescommonly used in the petroleum industry.

EXAMPLE 2

[0074] The oligomerization of the surfactant potassiumoctadec-1-ene-18-oate:

CH₂═CH(CH₂)₁₆CO₂ ⁻K⁺

[0075] in an aqueous solution was achieved using the followingprocesses. The viscoelastic surfactant solution was formed by mixing 60g/l potassium oleate with 60 g/l potassium chloride. A sample of 100 mlof the viscoelastic surfactant solution was purged with oxygen-free, drynitrogen and mixed with 10 mg of the initiator2,2′-azo(bisamidinopropane) dihydrochloride. The solution is heated at60° C. for 30 minutes under an atmosphere of oxygen-free, dry nitrogen.The resulting solution of oligomerized surfactants was about equallyviscoelastic as the original monomeric solution but the observedviscoelasticity was insensitive to contact with hydrocarbon. Theresulting oligomer can be thought of as related to oligo-ethylene, arelatively short-chained polyethylene, to which relatively long pendantgroups—(CH₂)₁₆CO₂ ⁻K⁺— are linked. The gel formed by the oligomerizedsurfactant retained its viscoelasticity after prolonged contact withwater. The observations led to the conclusion that the oligomer wasviscoelastic in character, much as was the original un-oligomerizedviscoelastic surfactant solution; but the oligomerized solution was morestable than the original un-oligomerized solution.

EXAMPLE 3

[0076] The oligomerization of a long-chain vinyl surfactant, thepotassium salt of octadeca-1,3-diene-17-oate:

CH₂═CH—CH═CH—(CH₂)₁₄CO₂ ⁻K⁺

[0077] in a viscoelastic solution was carried out at a concentration of60 g/l in the presence of 40 g/l ammonium chloride. The surfactantmonomers were oligomerized using 10 mg of the free radical initiator2,2′-azo(bis-amidinopropane) dihydrochloride in 100 ml of viscoelasticsurfactant solution which had been purged with oxygen-free, dry nitrogengas. The solution was heated at 60° C. for 30 minutes under anatmosphere of oxygen-free, dry nitrogen. The resulting oligomer can bethought of as related to oligo-butadiene, a relatively short-chainedpolybutadiene, to which relatively long pendant groups—(CH₂)₁₄CO₂ ⁻K⁺—are linked. Oligomerization of the surfactant resulted in a semi-rigidgel that retained the viscoelasticity of the original monomericsurfactant solution but showed none of its sensitivity to contact withhydrocarbon or water. The observations led to the conclusion that theoligomer was viscoelastic in character, much as was the originalun-oligomerized viscoelastic surfactant solution; but the oligomerizedsolution was more stable than the original un-oligomerized solution.

EXAMPLE 4

[0078] Oligomers of N-(undec-11-enyl)-N,N-dimethylammonium chloride weresynthesized by linking the head groups with a branched C₉ bridge. First,5-methylnonane-1,8-diamine was reacted with 11-chloroundec-1-ene,CH₂═CH(CH₂)₉Cl, and subsequently quaternized to produce the followingmonomer:

[CH₂═CH(CH₂)₉](CH₃)₂N⁺Cl⁻[(CH₂)₄CHCH₃(CH₂)₄]N^(+Cl)⁻(CH₃)₂[(CH₂)₉CH═CH₂]₃.

[0079] When this monomer was oligomerized, the product was a surfactantoligomer in which the N⁺Cl⁻ head groups, again, are almost completelyfree relative to their neighboring N⁺Cl⁻ head groups—each being linkedto each other through relatively long (CH₂)₄CHCH₃(CH₂)₄ chains or to theoligo-ethylene backbone through relatively long pendant (CH₂)₉ chains.

[0080] The preparation of the monomer described above began with5-methylnonane-1,8-diamine. This precursor was reacted with11-chloroundec-1-ene, CH₂═CH(CH₂)₉Cl, under conditions such that eachdiamine group was be converted to secondary amines. In this first stepin the synthesis, incompletely reacted material was separated from thedi-secondary amine by distillation. The small part of the5-methylnonane-1,8-diamine that had reacted too much and had becometertiarized or quaternized at one or both ends by excessive reactionwith the 11-chloroundec-1-ene were separated by fractionalcrystallization from the desiredN,N′-di-(1-decenyl)-5-methylnonane-1,8-diamine. Finally, this productwas then reacted with an excess of methylbromide and quaternized toproduce the monomer stated above. The oligomerization of this monomer:

[CH₂═CH(CH₂)₉](CH₃)₂N⁺Cl⁻[(CH₂)₄CHCH₃(CH₂)₄]N⁺Cl⁻(CH₃)₂[(CH₂)₉CH═CH₂]₃

[0081] in a viscoelastic solution was carried out at a concentration of60 g/l in the presence of 40 g/l ammonium chloride. A volume of 100 mlof the viscoelastic surfactant solution was placed in a bottle purgedwith oxygen-free, dry nitrogen gas to remove any dissolved oxygen. Aftersufficient purging 10 mg of the free radical initiator2,2′-azo(bis-amidinopropane) dihydrochloride was added to theviscoelastic surfactant solution and mixed thoroughly. The surfactantsolution was heated at 60° C. for 30 minutes under an atmosphere ofoxygen-free, dry nitrogen. Oligomerization of the surfactant monomers inmicelles resulted in the viscosity of the gel becoming largelyinsensitive to contact with hydrocarbon. The viscosity of the surfactantgel was not materially altered by the oligomerization of the surfactantmonomers. The oligomerized surfactant gel retained its gel strengthafter prolonged contact with water. The observations led to theconclusion that the oligomer was viscoelastic in character, much as wasthe original un-oligomerized viscoelastic surfactant solution; but theoligomerized solution was more stable than the original un-oligomerizedsolution.

EXAMPLE 5

[0082] The oligomerization of the viscoelastic surfactant monomer, thesodium salt of N-N-dimethyl-N-methylcarboxylate-N-1-hepten-7-ammoniumchloride, was carried out to giveoligo(1-hepten-7-quaternary-ammonio-N-N-dimethyl-N-methylcarboxylate),sodium salt. The resulting oligomer is believed to have the simplifiedstructure as indicated below in the acid form rather than thesodium-salt form:

[0083] in which x will have a value from about 2 to several hundredthousand, preferably from about 2 to several dozen, and more preferablyfrom about 2 to perhaps 4. The monomer was prepared by the reaction ofN-1-hept-6-enyl-N,N-dimethylamine with chloroacetic acid to produceN-1-hept-6-enyl-N-methylcarboxylic acid-N,N-dimethylammonium chloride.Upon neutralization with sodium hydroxide, the final product was thezwitterionic betaine which is the sodium salt ofN-hexadecyl-N-carboxymethyl-N,N-dimethylammonium chloride—it has anegative charge on the carboxyl group and the sodium cation associatedwith it as a counter ion, and a positive charge on the quaternary aminegroup and the chloride anion associated with it as a counter ion.

EXAMPLE 6

[0084] The viscoelastic monomer, the sodium salt ofN-carboxymethyl-N,N-dimethyl-N-1-hepten-7-ammonium chloride was mixedinto the solution of the conventional rod-shaped or spaghetti-like orspiraling-cylinder-like micelles of the viscoelastic surfactant that isthe sodium salt of N-hexadecyl-N-carboxymethyl-N,N-dimethylammoniumchloride. This illustrative monomer, the sodium salt ofN-carboxymethyl-N,N-dimethyl-N-1-hepten-6-ammonium chloride, differsonly in minor ways from the sodium salt ofN-hexadecyl-N-carboxymethyl-N,N-dimethylammonium chloride. Accordingly,the monomer was readily subsumed into the conventional rod-shaped orspaghetti-like micelles, whereupon oligomerization was initiated toproduce the sodium salt ofoligo-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate).This oligomer is inherently hydrophilic in its zwitterionic functionalgroups and hydrophobic in the hydrocarbon chains that link all thezwitterionic functional groups to each other. The oligomer is believedto be structurally quite similar to the viscoelastic surfactantmolecules in the well fluid and therefore is readily soluble in the wellfluid solution. The oligomers are likewise soluble or dispersible inother viscoelastic surfactant solutions such as 10% XE862 (a productthat is commercially available from Schlumberger) solution with 0.3%SDBS in 13.5 pound per barrel CaBr₂-based brine. Upon mixing theoligomer or polymer with the viscoelastic-surfactant-based fluid, theoligomer or polymer is believed to be subsumed into the rod-shaped orspaghetti-like or spiraling-cylinder-like micelles, whereupon, sittinginside these rod-shaped or spaghetti-like or spiraling-cylinder-likemicelles is one or more such oligomer or polymer molecules and whenconfigured in this fashion, the oligomer or polymer molecules impartgreater thermal stability to the micelles, greater resistance to shearstress and other stress conditions acting upon a fluid losspill—including, for example, exposure to oil, high shear in pumping andplacement, high temperature, high differential pressure, and low pH.

EXAMPLE 7

[0085] In another illustrative embodiment of the claimed subject matter,a sufficient quantity, about 10 volume %, of the co-monomer 1-heptenewas mixed into a viscoelastic solution of the sodium salt of1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate chloride,causing the solution to lose most or all of its viscoelastic character.Then the 1-heptene was co-oligomerized with the sodium salt of1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate chloride toproduceoligo-(1-heptene-co-1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate).The resulting oligomer is believed to have the simplified structure asindicated below:

[0086] in which x and y will have values dependent upon the molar ratioof reactants added to the oligomerization reaction and the sum of x andy will have a value of about 2 to several hundred thousand, preferablyfrom about 2 to several dozen, and more preferably from about 2 to ten.While this oligomer is inherently hydrophilic in its zwitterionicfunctional groups, there are two sources of hydrophobicity: in the1-heptene copolymer species and in the hydrocarbon chains that link allthe zwitterionic functional groups to each other. By varying the molarratio of the monomers present during the polymerization reaction, thehydrophobicity and viscoelastic surfactant properties of the resultingoligomer and polymer may be controlled. The oligomer contains a point ofstructural similarity to the small molecule viscoelastic surfactants andtherefore is readily soluble in such conventional viscoelasticsurfactant solutions as the 10% XE862 solution with 0.3% SDBS in 13.5pounds per gallon CaBr₂-based brine discussed above. Upon admixing theoligomer with the viscoelastic-surfactant-based fluid, the oligomer isbelieved to be subsumed into the rod-shaped or spaghetti-like orspiraling-cylinder-like micelles, whereupon, sitting inside theserod-shaped or spaghetti-like or spiraling-cylinder-like micelles is oneor more such oligomer molecules and when configured in this fashion, theoligomer molecules impart greater thermal stability to the micelles,greater resistance to shear stress and other stress conditions actingupon a fluid loss pill—including, for example, exposure to oil, highshear in pumping and placement, high temperature, high differentialpressure, and low pH.

EXAMPLE 8

[0087] In another illustrative embodiment of the claimed subject matter,a sufficient quantity, about 8 volume %, of the co-monomer 1-heptene wasmixed into a viscoelastic solution of 1-heptene-6,7-diol and the sodiumsalt of 1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonatechloride, causing the solution to lose most or all of its viscoelasticcharacter. The resulting oligomer,oligo-(1-heptene-co-1-heptene-6,7-diol-co-1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate),is believed to have the simplified structure as indicated below:

[0088] in which x, y and z will have values dependent upon the molarratio of reactants added to the oligomerization reaction and the sum ofx, y, and z will have a value of about 3 to several hundred thousand,preferably from about 3 to several dozen, and more preferably from about3 to ten. As with the other polymers of the claimed subject matter, theabove illustrative polymer is soluble in conventional viscoelasticsurfactant well fluids such as the 10% XE862 solution with 0.3% SDBS in13.5 pounds per gallon CaBr₂-based brine discussed above. Upon admixingthe illustrative polymer with such a viscoelastic-surfactant-basedfluid, the polymer is believed to be subsumed into the rod-shaped orspaghetti-like or spiraling-cylinder-like micelles, whereupon, sittinginside these rod-shaped or spaghetti-like or spiraling-cylinder-likemicelles is one or more such polymer molecules and when configured inthis fashion, the polymer molecules impart greater thermal stability tothe micelles, greater resistance to shear stress and other stressconditions acting upon a fluid loss pill—including, for example,exposure to high shear in pumping and placement, high temperature, highdifferential pressure, and low pH. Additionally, the co-monomer1-heptene-6,7-diol incorporates a vicinal diol functionality into thepolymer. Surface forces should cause the vicinal diol functionality topresent itself at the outer surface of any rod-shaped micelle in whichit has become subsumed. The vicinal diol functionality was subsequentlycrosslinked with borate, leading effectively to the crosslinking of theviscoelastic surfactant assemblies in which the oligomers had beensubsumed.

[0089] As discussed above, the novel oligomers or polymers taught inaccordance with the claimed subject matter contain chemical functionalgroups that are similar to those found in conventional viscoelasticsurfactants and thus are highly compatible with conventionalviscoelastic surfactant systems. Accordingly, the claimed subject matterteaches blends of the viscoelastic oligomers or polymers withconventional viscoelastic surfactant systems. Theviscoelastic-surfactant-based fluids useful for the claimed subjectmatter are those in the following citations, all of which areincorporated herein by reference—Canadian Patent 1,298,697, U.S. Pat.Nos. 4,615,825, 4,695,389, 4,725,372, 5,258,137, 5,551,516, 5,691,292,5,964,295, 5,965,502, 5,979,555, 5,979,557, 6,140,277, 6,194,355,6,194,356, 6,211,120, 6,232,274, 6,239,183, Paper SPE 17,168, Paper SPE30,098, Paper SPE 30,114, Paper SPE 30,458, Paper SPE 31,114, Paper SPE38,622, Paper SPE 56,467, Paper SPE 57,432, Paper SPE 59,478, and PaperSPE 60,322.

[0090] Furthermore, the oligomeric or polymeric viscoelastic surfactantsmay be added to a well fluid in substantially any convenient manner.Thus, the oligomeric or polymeric viscoelastic surfactants may be addeddirectly to the well fluid either in solid form or in the form of anaqueous solution. Alternatively, the oligomeric or polymericviscoelastic surfactants may be separately added to a solution alreadycontaining other surfactants or viscoelastic surfactants to provide afluid loss controlling base solution, with the optional crosslinkingagents thereafter being added to the fluid loss controlling basesolution immediately prior to use.

[0091] In preferred embodiments, the oligomeric or polymericviscoelastic surfactants, including optional crosslinking agents, aresupplied to the well fluid at a level of about 20 ppm to about 20 partsper 100 parts of the well fluid, more preferably about 100 ppm to about15 parts per 100 parts of the well fluid.

[0092] In more preferred embodiments, the oligomeric or polymericviscoelastic surfactants, including optional crosslinking agents, aresupplied to the well fluid already containing an amount of othersurfactants or viscoelastic surfactants. When these more preferredembodiments are exercised, the oligomeric or polymeric viscoelasticsurfactants are supplied at a level of about 20 ppmw to about 10 partsper 100 parts of the well fluid, preferably about 100 ppmw to about 5parts per 100 parts of the well fluid.

[0093] Techniques for measuring fluid loss control are well known in theart and should be well known to one of skill and knowledge of theformulation of drilling fluids. Specifically, the fluid lossmeasurements of the present disclosure were made with reference to APIRecommended Practice RP 13B-1, Second Edition, September 1997, pp. 9-11,the contents of which are incorporated by reference.

[0094] While the apparatus, compositions and methods of the claimedsubject matter have been described in terms of preferred or illustrativeembodiments, it will be apparent to those of skill in the art thatvariations may be applied to the process described herein withoutdeparting from the concept and scope of the claimed subject matter. Allsuch similar substitutes and modifications apparent to those skilled inthe art are deemed to be within the scope and concept of the claimedsubject matter.

What is claimed is:
 1. A wellbore fluid comprising: an aqueous basedcontinuous phase; a viscoelastic surfactant; and a surfactant-polymercompound soluble in an aqueous solution, the surfactant-polymer compoundhaving a hydrophobic backbone and a plurality of hydrophilic functionalgroups attached to the hydrophobic backbone, wherein the hydrophobicbackbone is the reaction product of one or more molecules havingpolymerizable alkene or alkyne functional groups; wherein thehydrophilic functional groups are selected from: zwitterionic surfactantfunctional groups, anionic surfactant functional groups, cationicsurfactant functional groups, and nonionic surfactant functional groups;and, wherein the combination of the viscoelastic surfactant andsurfactant-polymer compound form micellar assemblies.
 2. The wellborefluid of claim 1 further comprising a water-soluble inorganic salt. 3.The wellbore fluid of claim 1 wherein the acid form of thesurfactant-polymer compound has the structure:

wherein x=2 to 300,000.
 4. The wellbore fluid of claim 3 wherein x=2 to36.
 5. The wellbore fluid of claim 1 wherein the surfactant-polymercompound is a salt of oligo- or poly-(α-alkenyl -ω- or α-alkynyl-ω-quaternary-ammonio-N-N-dialkyl-N-alkylcarboxylate) or a mixturefurther comprising a salt ofN-alkyl-N-carboxymethyl-N,N-dimethylammonium chloride.
 6. The wellborefluid of claim 1 wherein the surfactant-polymer compound is a salt ofoligo- orpoly-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate) ora mixture further comprising a salt ofN-hexadecyl-N-carboxymethyl-N,N-dimethylammonium chloride.
 7. Thewellbore fluid of claim 1 wherein the surfactant-polymer compound is asalt of oligo- or poly-(α-alkenyl -ω-or α-alkynyl-ω-quaternary-ammonio-N,N-dialkyl-N-alkylcarboxylate).
 8. The wellborefluid of claim 1 wherein the surfactant-polymer compound is a salt ofoligo- orpoly-(1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-methylcarboxylate).9. A wellbore fluid comprising: an aqueous fluid; a viscoelasticsurfactant; a thickener soluble in the aqueous fluid, wherein thethickener has a hydrophobic oligomeric or polymeric backbone made fromthe reaction of alkene monomer or alkyne monomer, or mixtures thereof,and wherein surfactant functional groups are attached to the hydrophobicbackbone, wherein the surfactant functional group is selected from:zwitterionic surfactant functional groups, anionic surfactant functionalgroups, cationic surfactant functional groups, and nonionic surfactantfunctional groups; and, wherein the thickener has a molecularconfirmation such that the surfactant functional groups are hydrophilicand the hydrophobic oligomeric or polymeric backbone is hydrophobic; andwherein the combination of viscoelastic surfactant and thickener formmicellar assemblies such that the wellbore fluid thickener developsviscoelastic characteristics.
 10. The wellbore fluid of claim 9 whereinthe thickener has the following structure:

wherein x+y=2 to 300,000.
 11. The wellbore fluid of claim 10 whereinx+y=2 to
 36. 12. The wellbore fluid of claim 11 wherein the thickener isa salt of oligo- or poly-(α-alkene -ω- or α-alkyne-co-α-alkenyl -ω- orα-alkynyl -ω-quaternary-ammonio-N,N-dialkyl-N-alkylsulfonate).
 13. Thewellbore fluid of claim 11 wherein the thickener is oligo- orpoly-(1-heptene-co-1-hepten-7-quaternary-ammonio-N,N-dimethyl-N-propylsulfonate).14. The wellbore fluid of claim 9 wherein the viscoelastic surfactant isselected from the following: anionic, nonionic, amphoteric,zwitterionic, alcohols, alkano-lamides, alkanolamines, alkylarylsulfonates, alkylaryl sulfonic acids, amine acetates, amine oxides,amines, sulfonated amines, sulfonated amides, betaines, block polymers,carboxylated alcohols, alkylphenol ethoxylates, carboxylic acids, fattyacids, ethoxylated alcohols, ethoxylated alkylphenols, ethoxylatedamines, ethoxylated amides, ethoxylated fatty acids, ethoxylated fattyesters, fatty esters, fluorocarbon-based surfactants, glycerol esters,glycol esters, heterocyclic surfactants, imidazolines, isethionates,lanolins, lechithins, methyl esters, monoglycerides, olefin sulfonates,phosphate esters, polyethylene glycols, polysaccharides, polyacrylicacids, polyacrylamides, propoxylated alcohols, propoxylatedalkylphenols, propoxylated amines, propoxylated amides, propoxylatedfatty acids, propoxylated fatty esters, protein-based surfactants,quaternary surfactants, sarcosamines, silicone-based surfactants, soaps,sodium isethionate, sorbitans, sucrose and glucose esters, sulfates andsulfonates of oils and fatty acids, sulfates and sulfonates ofethoxylated alkylphenols, sulfates of alcohols, sulfates of ethoxylatedalcohols, sulfates of fatty esters, sulfonates of hydrocarbons andpetroleum, sulfosuccinates, taurates, and tridecyl and dodecyl benzenesulfonic acids and mixtures thereof.
 15. The wellbore fluid of claim 9wherein the thickener has the following structure:

where R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, and t=1 to 16, u=6 to 12, v=1 to 18, w=1to 3, and x+y+z=3 to 300,000 and S₁=CO₂ ⁻ or SO₃ ⁻.
 16. The wellborefluid of claim 15 wherein t=12 to 16, u=6 to 12; v=12 to 18, w=1 to 3,x=0 to 10,000, y=2 to 300,000 and z=0 to 10,000
 17. The wellbore fluidof claim 9 wherein the oligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, andx+y+z=3 to 300,000.
 18. The wellbore fluid of claim 17 wherein t=12 to16, u=6 to 12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000,and z=0 to 10,000.
 19. The wellbore fluid of claim 9 wherein theoligomer or polymer has the following structure

where R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, x+y+z=3to 300,000, and S₁=CO₂ ⁻ or SO₃ ⁻.
 20. The wellbore fluid of claim 19wherein t=12 to 16, u=6 to 12, v=12 to 18, x=0 to 10,000, y=2 to300,000, and z=0 to 10,000.
 21. The wellbore fluid of claim 9 whereinthe oligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein, R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, w=1 to12, and x+y+z=3 to 300,000.
 22. The wellbore fluid of claim 21 whereint=12 to 16, u=6 to 12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to300,000, and z=0 to 10,000.
 23. The wellbore fluid of claim 9 whereinthe oligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, and t=1 to 16, u=6 to 12, v=1 to 18, w=1to 3, and x+y+z=3 to 300,000 and S₁=CO₂ ⁻ or SO₃ ⁻.
 24. The wellborefluid of claim 23 wherein t=12 to 16, u=6 to 12; v=12 to 18, w=1 to 3,x=0 to 10,000, y=2 to 300,000 and z=0 to 10,000
 25. The wellbore fluidof claim 9 wherein the oligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, andx+y+z=3 to 300,000.
 28. The wellbore fluid of claim 25 wherein t=12 to16, u=6 to 12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to 300,000,and z=0 to 10,000.
 27. The wellbore fluid of claim 9 wherein theoligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, x+y+z=3to 300,000, and S₁=CO₂ ⁻ or SO₃ ⁻.
 28. The wellbore fluid of claim 27wherein t=12 to 16, u=6 to 12, v=12 to 18, w=1 to 3, and x=0 to 10,000,y=2 to 300,000, and z=0 to 10,000.
 29. The wellbore fluid of claim 9wherein the oligomer or polymer has the following structure:

wherein R₁, R₂, R₃, R₄, R₅, R₆=H or CH₃

wherein, R₁₀, R₁₁, R₁₂=H or CH₃, t=1 to 16, u=6 to 12, v=1 to 18, w=1 to12, and x+y+z=3 to 300,000.
 30. The wellbore fluid of claim 29 whereint=12 to 16, u=6 to 12, v=12 to 18, w=1 to 3, and x=0 to 10,000, y=2 to300,000, and z=0 to 10,000.
 31. The wellbore fluid of claim 9 whereinthe oligomers or polymers are crosslinked with polyvalent metal ions,formaldehyde, or glutaraldehyde.
 32. The wellbore fluid of claim 31wherein the polyvalent metal ions are selected from the following: Fe²⁺,Cd²⁺, Co²⁺, Ca²⁺, Cu²⁺, UO₂ ²⁺, PbO²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ce³⁺, Ti⁴⁺,Zr⁴⁺, Sn⁴⁺ and mixtures thereof.
 33. A method of making a wellbore fluidcomprising blending: an aqueous fluid phase; a viscoelastic surfactant;a water-soluble inorganic salt; an oligomer or polymer soluble in anaqueous salt solution, the oligomer or polymer comprising a hydrophobicoligomeric or polymeric backbone made from the oligomerization orpolymerization of alkene or alkyne monomer groups, or mixtures thereof,the oligomer or polymer further comprising zwitterionic functionalgroups attached to the hydrophobic backbone, wherein the oligomer orpolymer is hydrophilic in the zwitterionic functional groups andhydrophobic in the backbone hydrocarbon chain to form micellarassemblies such that the oligomers or polymers develop viscoelasticcharacter prior to a polymerization step.
 34. The method of claim 33further comprising a polymerization step of the polymer or oligomer,then drying the product and subsequently admixing it into a solution ofconventional surfactants.
 35. A method of drilling a subterranean well,the method comprising: drilling the subterranean well using a rotarydrilling rig and circulating a drilling fluid in the subterranean well,wherein the drilling fluid is the wellbore fluid of claim
 1. 36. Amethod of reducing the loss of fluid out of a subterranean well, themethod comprising injecting into the subterranean well a wellbore fluidas recited in claim 1.